The present invention relates to a method of coating a non-conductive substrate.
Coating a substrate with a metal or a metal compound is widely conducted for corrosion protection, decoration, reinforcement and the like. The representative examples of prior art methods for coating a substrate include electroplating, vacuum deposition, and spraying.
Electroplating is a method of depositing a metal ion by an electrochemical reaction on an electrode dipped in a plating solution. This technique has disadvantages such as the types of coating materials being limited. In addition, only a coating in the order of a few microns is formed because a thick coating is difficult to form. In addition, electroplating is not an economical method because it requires a complicated large-scale system and a large amount of electric power so that production cost is high. When a plating solution containing cyanogen, sodium hydroxide, or ammonia is used, plating efficiency and recovery rate of a coating material are low and waste disposal of the plating solution causes serious problems on preservation of environment. In the case of melt plating, a melted coating material tends to react with a substrate to be coated because the coating treatment is conducted at a high temperature.
On the other hand, vacuum deposition is a method of vacuum coating either by heating a target material placed on a filament or kept in a crucible by a heating resistor, electron beam or laser light, or by ion-sputtering of a target material. Although laser-heating and ion-sputtering can be conducted at a relatively low temperature compared with other vacuum deposition techniques, they can not eliminate such disadvantages as a crucible causing contamination and coating materials reacting with each other or with a substrate so that an alloy is formed because the laser-heating and ion-sputtering are classified as high-temperature coating methods using thermal melting. In addition, since particles vacuum-evaporated or sputtered from a target are so active as to react with residual gas to provide impurities, a coating having high purity can not be obtained. Moreover, coating efficiency and recovery rate of a coating material are low. The coating obtained by this method has low adhesion to a substrate and hence is not highly durable. Furthermore, when a substrate having a large area is coated, a coating having uniform thickness can not be obtained.
Spray coating is a method of coating a substrate by spraying a coating solution from a nozzle onto a substrate. This method is simpler than the above two methods. However, spraying has the disadvantage that a coating has low adhesion to a substrate and low density. In addition, this method is not economical because it requires special steps such as pre-washing of the substrate surface, pre-treatments for providing the substrate with adherability to a coating, a drying step and the like.
If the substrate to be coated has a complicated shape, for example, if the inner surface of a hollow cylinder is to be coated, a uniform coating can not be obtained by using any of the above methods.